SrFe₁₂O₁₉ based ceramics with ultra-low dielectric loss in the millimetre-wave band

Non-reciprocal devices such as isolators and circulators, based mainly on ferromagnetic materials, require extremely low dielectric loss in order for strict power-link budgets to be met for millimetre (mm)-wave and terahertz (THz) systems. The dielectric loss of commercial SrFe₁₂O₁₉ hexaferrite was significantly reduced to below 0.002 in the 75-170 GHz band by thermal annealing. While the overall concentration of Fe²⁺ and oxygen vacancy defects is relatively low in the solid, their concentration at the surface is significantly higher, allowing for a surface sensitive technique such as XPS to monitor the Fe³⁺/Fe²⁺ redox reaction. Oxidation of Fe²⁺ and a decrease in oxygen vacancies are found at the surface on annealing, which are reflected in the bulk sample by a small change in the unit cell volume. The significant decrease in the dielectric loss property can be attributed to the decreased concentration of charged defects such as Fe²⁺ and oxygen vacancies through the annealing process, which demonstrated that thermal annealing could be effective in improving the dielectric performance of ferromagnetic materials for various applications.